Formation mechanism and mixing behavior of Nanyang thermal spring, Xingshan County of Hubei Province, central China

This study elucidates the hydrochemical characteristics and formation mechanisms of the Nanyang thermal spring, situated in a typical low-medium temperature geothermal field in central China. Based on isotopic and hydrochemistry analyses, the thermal groundwater was found to originate from precipitation via the Shennongjia group of mountains. In addition, hydrochemical features of the thermal groundwater derived from the regional flow system show it is generally of Na-Cl type, with high values of total dissolved solids and minor elements (Sr, F). This is different from the shallow cold water of the local flow system which is characterized by Ca-HCO₃ or Ca·Mg-HCO₃ type. The Na-K-Mg geothermometer indicates that none of the thermal groundwater reached water–rock equilibrium during pumping, while the mixing model shows that Nanyang thermal spring comprises approximately 84.9% shallow cold groundwater and 15.1% deep thermal groundwater. Furthermore, the reservoir temperature was evaluated by chemical geothermometry and validated by fluid-mineral equilibria calculations. The results show that quartz geothermometers, along with a silica-enthalpy mixing model, can provide a consistent reservoir temperature of 136.4–145.0 °C (average 141.8 °C) considering mixing behavior. Finally, a conceptual model of the formation housing the thermal spring was developed to highlight the origin-source pathway. The conceptual model shows that the thermal groundwater originates from long-distance flow of precipitation recharged via a deep-seated fault and fracture zone (mainly the Jiuchong Fault) from the Shennongjia Mountains, and then flows upwards to the surface, mixing with the local shallow cold groundwater as it encounters an impermeable shale layer.